Modern, high speed, automated concrete block plants and concrete paver plants make use of concrete block molds that are open at the top and bottom. These molds are mounted in machines which cyclically station a pallet below the mold to close the bottom of the mold, deliver dry cast concrete into the mold through the open top of the mold, densify and compact the concrete by a combination of vibration and pressure, and strip the uncured blocks from the mold by a relative vertical movement of the mold and the pallet.
For efficient high-volume production, concrete block molds are typically configured to produce multiple blocks simultaneously. A concrete block mold generally comprises side walls and end walls that define the periphery of a mold cavity. Within this mold cavity, division plates may be used to sub-divide the mold cavity into a plurality of block-forming cavities. Further, movable side walls may be used to form the side faces of the block-forming cavity. The division plates are generally rectangular-shaped plates attached to the side walls of the mold. Further, the side walls of the block cavity and the division plates may be covered with replaceable mold face linings to protect the mold components from abrasive wear.
As disclosed in U.S. Published Patent Application 20030182011, some blocks are now being formed with patterned or other processed front faces while retaining the high-speed, mass production of the blocks. As disclosed in U.S. Published Patent Application 20030182011, the blocks can be formed front face-up in the mold, allowing the front face of the block to be contacted by a stripper shoe which imparts a desired three-dimensional pattern to the front face. When a block is formed front-face-up in the mold, the top and bottom surfaces of the blocks (from the perspective of the block as laid in a wall) are formed by division plates. Because the side surfaces of a block must converge to allow the blocks to be laid up in a curved or radiused wall, the front of the block is typically wider than the rear of the block. In order for a block formed front-face-up to be discharged through the bottom of the mold, the side surfaces of a block must be formed by movable side walls that, in a first position during molding, form the wider front portion and narrower bottom portion of the block, and in a second position during discharge of the block from the mold, move sufficiently out of the way for the wider front portion of the block to pass through the bottom of the mold.
A problem that arises when blocks are formed front-face-up in a conventional block mold is that the blocks are prone to being formed with the top and bottom surfaces not being flat and parallel to each other. Since concrete retaining wall blocks are typically assembled without mortar, there is little ability to accommodate variations in the flatness and parallelism of the top and bottom surfaces during the assembly of a wall. It is very important, therefore, that the top and bottom surfaces of the blocks that engage with other blocks be formed as flat as possible and parallel to each other to allow the blocks to lay flat and level on blocks in a lower course of blocks, as well as to allow blocks in an upper course to lay flat and level.
It is also important during the commercial manufacture of concrete blocks that the manufacturing expenses be minimized. Certain components in the concrete mixture are more expensive, such that increasing the percentage of those components in the concrete mixture increases the manufacturing expense. In particular, cementitious materials are a component of a concrete mixture that is typically more expensive than other components. However, the percentage of cementitious materials in the concrete mixture affects the stability and dimensional control of the resulting concrete block. Therefore, it is desired to minimize the amount of cementitious material required in the concrete block mixture while still maintaining acceptable block properties and dimensional control.
Thus, there is a demand for concrete block manufacturing processes that provide for improved control of the flatness and parallelism of the top and bottom surfaces of concrete blocks formed front face-up in a mold, while minimizing the expense of the concrete mixture.